Resin compositions for cationic electrocoatings each comprising, as the main components, a polyamine resin (e.g. amino-epoxy resin adduct) and a blocked polyisocyanate compound have been used in a large amount for their excellency in corrosion resistance, etc. These compositions, however, have various problems as listed below and solutions thereof are required strongly.
(1) They begin to cure at a temperature of 170.degree. C. or more, which is too high.
(2) When they are heated at high temperatures, the blocked polyisocyanate compound causes thermal decomposition, which generates a resinous tar and soot, allows the top coating film to give rise to yellowing, bleeding and insufficient curing, gives significantly reduced weather resistance and tends to cause whitening.
(3) When an organotin compound is added to them as a catalyst for lowering the initial temperature of curing, the compound poisons catalysts for exhaust gas combustion in some cases.
Resins for self-crosslinking electrocoatings using no curing agent having the above-mentioned problem are also known and were proposed in, for example, Japanese Patent Publication No. 23807/1974 (GB-A-1327071), Japanese Patent Publication No. 31736/1974 (GB-A-1306101, GB-A-1306102), Japanese Laid-Open Patent Application No. 13432/1972 (US-A-3975346) and Japanese Laid-Open Patent Application No. 69896/1973 (US-A-3937679). All of these resins have a problem that they do not satisfy both of the bath stability and film curability of electrocoating. Specifically explaining, most common epoxy compounds of glycidyl ether type, for example, bisphenol A glycidyl ether and novolac phenyl polyglycidyl ether have excellent curability but inferior bath stability.
A composition using, as the curing agent, an epoxy resin having an alicyclic skeleton and/or a bridged alicyclic skeleton, proposed in Japanese Laid-Open Patent Application No. 255874/1990 (EP-A-356970), achieved most or-the objects intended therein but is insufficient in film properties when baking was conducted at low temperature for a short period of time.
The present inventors made a further study with a main aim of developing a resin composition for aqueous coating useful particularly in cationic electrocoatings, which does not use any of a blocked polyisocyanate compound, an organotin compound, etc. and which is free from the above-mentioned problems.
As a result, the present inventors found that a resin composition for aqueous coating, which comprises a particular novolac-substituted phenol type epoxy resin as a curing agent, has improved storage stability in electrocoating bath owing to the reduced reactivity with acid or hydroxyl group and, as compared with epoxy resins containing an alicyclic skeleton and/or a bridged alicyclic skeleton, has excellent low-temperature curability. This finding has led to the completion of the present invention. According to the present invention, it is not necessary to use any of a blocked polyisocyanate compound, an organotin compound, etc. and all of the abovementioned problems caused by the use of said compounds can be solved.